1. Field of the Invention
The present invention relates to improvements on a Mode S radar which identifies an aircraft by receiving a reply to a Mode-S interrogation having been transmitted to a Mode-S transponder installed in the aircraft, and by then decoding a signal of the reply.
2. Description of the Related Art
Radars for aircraft surveillance, which are used for air traffic control over air routes and airspaces surrounding airports, are broadly divided into a primary surveillance radar (PSR) and a secondary surveillance radar (SSR).
The secondary surveillance radar (SSR) is designed to obtain various kinds of information on an aircraft by transmitting interrogations from an interrogator placed on the ground toward a transponder installed in the aircraft, and by then receiving and decoding replies from the aircraft to the interrogations.
Interrogations of the secondary surveillance radar are categorized into Mode A, Mode C and Mode S on the basis of kinds of information to be obtained. Mode A is used for obtaining identification (ID) information on an aircraft, which information is expressed in the form of a 4-digit number, while Mode C is used for obtaining altitude (ALT) information on an aircraft.
Mode S is capable of obtaining Mode-A code information, altitude information, route information and velocity information on an aircraft by using a selective interrogation, in addition to a 24-bit address information and positional information on the aircraft, which are obtained by using a Mode S only all-call interrogation. Moreover, Mode S has a ground-to-air data link communications function.
As described in “Secondary Surveillance Rader for Air Traffic Control—SSR Mode S” edited by Yoshio Hashida, Hisashi Ohtomo and Yoshinori Kuji (Toshiba Review Vol. 59 No. 2 (2004), pp. 58-61), the Mode S radar transmits a Mode S only all-call interrogation and Mode S selective interrogations so as to accomplish an initial acquisition of an aircraft equipped with a Mode S transponder and ordinary surveillance following the initial acquisition. Additionally, a parity checking function is incorporated in each of Mode S interrogation signals and reply signals so that each of the reply signals can be decoded with a bit error rate of 10−7.
A conventional Mode S radar is configured as shown in FIG. 1. The Mode S radar includes a transmission/reception antenna 1, a transmission/reception switch 2, a transmitter 3, a receiver 4, a signal processor 5, a correlation processor 6 and a transmission controller 7. The antenna 1 is capable of performing rotational scans. The transmission/reception switch 2 is connected to the antenna 1. The transmitter 3 and the receiver 4 which are connected to the transmission/reception switch 2. The signal processor 5 is connected to the transmitter 3 and the receiver 4, and schedules interrogations and replies. The correlation processor 6 is connected to the signal processor 5, and the transmission controller 7 is connected to the correlation processor 6. Here, a so-called interrogator is composed of the transmitter 3, the receiver 4, the signal processor 5 and the correlation processor 6.
Mode S interrogations scheduled by the signal processor 5 include a Mode S only all-call interrogation and Mode S selective interrogations. Each of the Mode S interrogation thus scheduled is transmitted toward an aircraft (that is, an aircraft equipped with a Mode S transponder) A through the transmitter 3, the transmission/reception switch 2 and the antenna 1. The thus transmitted interrogation is received by a transponder A2 through an aerial A1 of the aircraft A. A Mode S reply from the transponder A2, which corresponds to the Mode S interrogation from the Mode S radar, is transmitted toward the antenna 1 through the aerial A1 in return.
The mode S reply received by the antenna 1 is fed to the signal processor 5 sequentially through the transmission/reception switch 2 and the receiver 4 so as to be decoded by the signal processor 5. Meanwhile, it is judged whether or not the thus decoded reply should be subjected to processing in the own radar site, and the reply judged to be subjected to the processing is fed to the correlation processor 6. The correlation processor 6 includes an operation processing circuit 61 and a memory (a memory circuit) 62 which are formed of a computer, generates a detection report (that is, a target message) on the aircraft A by operation processing accompanied with storing into and reading out from the memory 62, and then feeds the detection report to the transmission controller 7.
An operation procedure of the abovementioned conventional Mode S radar will be described on the basis of a flowchart shown in FIG. 2 and with reference to a block diagram in FIG. 1 and a sequence of interrogations and replies which is shown in FIG. 3. As shown in FIG. 2, the Mode S radar transmits a Mode S only all-call interrogation (UF=11) at constant intervals through an entire cycle in every scan (step S101), and the aircraft A (the mode S transponder) returns a reply upon reception of the Mode S only all-call interrogation. A Mode S address and positional information are obtained from each of replies (DF=11) received from the aircraft A which are obtained in a plurality of scans (the first scan and the second scan) so as to judge whether or not initial acquisition has been accomplished by the correlation processor 6 (step S102). In general, the initial acquisition of the aircraft A is performed on the basis of a correlation between tracking data obtained from two successive scans, and data on a position (a distance and a direction) and data on a unique Mode S address of the aircraft A.
When judgment is YES in step S102, that is, when the initial acquisition is accomplished, the signal processor 5 schedules and transmits Mode S selective interrogations (UF=4 and UF=5) in order to obtain altitude information and Mode A code information on the aircraft A in a scan (a third scan) following the initial acquisition. Thereafter, the signal processor 5 obtains positional information including the altitude information on the aircraft A, and the Mode A code information, and then feeds the information to the operation processing circuit 61 of the correlation processor 6 (step S103).
When judgment is NO in step S102, that is, when the initial acquisition is not accomplished, detection of replies (DF=11) corresponding to the Mode S only all-call interrogation (DF=11) in step S101 is always executed, and the process moves on to step S103 only after the initial acquisition has been performed on the basis of the plurality of scans.
After step S103, positional information and altitude information are obtained with a Mode S selective interrogation (UF=4), a detection report on the aircraft A is generated from these information, and data of a Mode A code stored in the memory 62 of the correlation processor 6, and is transmitted toward an unillustrated air traffic control system through the transmission controller 7 (step S104).
In order to continuously surveil the aircraft A in each of scans (fourth and later scan) following the scan (the third scan) of step S103, the MODE S RADAR transmits the Mode S selective interrogation (UF=4) used for obtaining positional information including altitude information, and receives and decodes a reply from the aircraft A corresponding to the thus transmitted Mode S selective interrogation (UF=4). Meanwhile, the correlation processor 6 generates a detection report on the basis of the positional information on the aircraft A, which includes the altitude information thereof, having been obtained by thus decoding the reply, and also on the basis of the data of the Mode A code having been previously obtained and stored in the memory 62, and transmits the detection report toward the unillustrated air traffic control system through the transmission controller 7 (step S106).
In this manner, in the conventional Mode S radar, data of the Mode A code obtained in surveillance in the initial scan (the third scan) immediately after the initial acquisition is stored in the memory 62. In addition, the detection report is generated in surveillance in each of the subsequent scans (the fourth and later scans) by reading out and utilizing the Mode A code thus stored in the memory 62. Accordingly, in each of the abovementioned fourth and later scans, a Mode A code requesting interrogation (UF=5) and a reply thereto are omitted. For this reason, the necessity of using an RF channel between the Mode S radar and the aircraft A in connection with the otherwise transmitted Mode A code requesting interrogation (UF=5) is eliminated. As a result, utilization of the RF channel can be expanded into data link communications and the like, and furthermore, more efficient utilization of a beam dwell time can be achieved.
In the above description, it has been described, that the mode A code obtained in the first scan (the third scan) after the initial acquisition is stored, and that reading and utilization of the thus stored mode A code is continued in the subsequent scans. However, note that, even for the mode A code, which is previously set for the aircraft before takeoff, and which basically is not changed, the Mode S code selective interrogation (UF=5) used for obtaining the Mode A code is carried out, for example, at the time when the flight status information has changed due to an Alert status brought about with a Mode A code altering operation by a pilot, or when the tracking of the aircraft A is lost midway through the tracking.
Additionally, although the above described Mode A code has been referred to as ID of a Mode-S-transponder-equipped aircraft, that is, the identification number of a Mode-S-transponder-equipped aircraft, it is sometimes referred to as a discrete beacon code (DBS).
As has been described above, in the conventional Mode S radar, the Mode A code obtained in the initial scan (the third scan) immediately after the accomplishment of the initial acquisition of the aircraft A in the plural scans (the first and second scans) is stored in the memory 62. Additionally, in the surveillance of the aircraft A in each of the fourth and later scans, the stored Mode A code is read out every time when necessary as long as there is no change in flight status of the aircraft, and is utilized in the generation of a detection report (a target message) along with the altitude information and the positional information which are acquired with the Mode S selective interrogation (UF=4).
However, unlike surveillance by using Mode A/C, a Mode A code necessary for detection report generation in the Mode S secondary surveillance is obtained by reading out the data, which has been obtained in one scan after the accomplishment of the initial acquisition, and which has been then stored in the memory. For this reason, there has been the following problem. Specifically, if there is an error in the stored data of the Mode A code, a serious adverse effect occurs on air traffic control because detection reports are continuously generated, even in the MODE S RADAR having an error rate of 10−7.
In a case where the Mode S selective interrogation for obtaining the Mode A code is supposed to be carried out in every scan as in the case with a conventional Mode A/C radar in order to avoid this problem, incorrect Mode A code data cannot be outputted over a long period of time even if the incorrect Mode A code data has been detected as a normal code for a rare occasion. However, in a system having such a configuration, the number of interrogations and replies to and from the same Mode-S-transponder-equipped aircraft is double that in the current system. For this reason, the occupation time of RF channel is increased, and therefore, when efficient utilization of data links which will be prevalent in the future, is taken into consideration, a harmful effect of decreasing a channel time allocated to exchanges of data link information is expected. Accordingly, there has been a problem of causing an adverse effect on adequate exchanges of user data between air and ground.